Abstract

A theory for identifying the chemical composition of large size-distributed particulates by a tunable CO₂ laser lidar is developed. The single-scatter lidar equation is used to model lidar returns from a particulate cloud embedded in an otherwise clear atmosphere. The backscatter and extinction coefficients that appear in the lidar equation are described by asymptotic expressions that are valid for ensembles of particulates that are large with respect to CO₂ laser wavelengths. Several distribution functions including the lognormal and modified gamma distributions are used to describe the particle size dispersion of the particulates. The accuracy of the asymptotic backscatter and extinction coefficients relative to an exact Lorenz-Mie calculation is investigated. The asymptotic extinction coefficient is shown to depend only on the size distribution of the particulates and contains no information about the chemical composition of the particulates. The asymptotic backscatter coefficient is expressed in terms of the Fresnel reflectance for normal incidence and thus carries information about the chemical composition of the particulates.

© 1986 Optical Society of America